Time domain metal detectors usually synchronously demodulate (or sample) a receive signal from a receive coil commencing after a short delay following a voltage transition of a transmit signal, for example, after the back emf of a pulse induction (PI) transmit pulse, or after a switch from a high voltage (e.g. 200V) to a low-voltage (e.g. 5V) or to zero voltage. The short delay is to avoid the processing electronics from demodulating potential spurious interfering signals (unwanted signals) from mildly conductive soils. Typically, these signals from mildly conductive soil have signal components of particularly short duration; they will decay very rapidly. Detection of these signals is undesirable in a metal detector. Given that they were the only difficulty to be overcome, a short delay of the onset of demodulation after the transition of transmit signal would be sufficient to reduce their effect upon detection. In most detectors, however, the demodulation must be delayed by more than is required to remove the effect of the signals from mildly conductive ground. In most detectors, the minimum of the practical delay is usually limited by the time constant of the critically damped receive coil, either because the decaying transient signal components in the receive coil, due to a voltage transition of the transmit signal, contain signals related to the commonly major soil component known in the field of metal detection, namely the signal components due to soil reactive magnetic permeability, which is normally required to be nulled out, or to prevent said transients from causing receive signal electronic to overload, or both.
The required delay before demodulation, (or sampling), is undesirable for the detection of fast constant metal targets, because much of the decaying signal from the fast time constant targets following a transition of the transmit signal occurs during the delay, and thus most of this target signal is not added into the receive demodulation. Many sought buried metal targets have short time constants: for example; some minimum metal land mines, small gold nuggets, and fine jewellery. Improving the sensitivity to very short time constant targets requires reduction of the delay before demodulation, in turn requiring reduction of the time constant of the critically damped receive coil. Hitherto, this was usually achieved by choosing a low inductance for the receive coil, or eliminating the capacitive loading of the receive coils by the capacitance of the connecting cable, through installation of the receive preamplifier within the coil housing.